iOS Pods: introduced fb-only __generated__ dir

Summary:
To help cache Pods artifacts for Sandcastle test run, create a `__generated__` dir under RNTester/Pods, which will contain the normally-top-level files produced by `pod install`. The files will then be symlinked by the script.

allow-large-files

Reviewed By: axe-fb

Differential Revision: D17470686

fbshipit-source-id: 50e4c57e913e2884eed27db94c50181fe4632133

.gitignore

@@ -83,15 +83,15 @@ RNTester/build
 /RNTester/Gemfile.lock
 
 # RNTester Pods: all top level files of the Pods folder must be listed
-/RNTester/Pods/DoubleConversion
-/RNTester/Pods/Folly
-/RNTester/Pods/Headers
-/RNTester/Pods/Manifest.lock
-/RNTester/Pods/Local Podspecs
-/RNTester/Pods/Pods.xcodeproj
-/RNTester/Pods/Target Support Files
-/RNTester/Pods/boost-for-react-native
-/RNTester/Pods/glog
+RNTester/Pods/DoubleConversion
+RNTester/Pods/Folly
+RNTester/Pods/Headers
+RNTester/Pods/Manifest.lock
+RNTester/Pods/Local Podspecs
+RNTester/Pods/Pods.xcodeproj
+RNTester/Pods/Target Support Files
+RNTester/Pods/boost-for-react-native
+RNTester/Pods/glog
 
 # react-native-codegen
 /ReactCommon/fabric/components/rncore/